The xylenes, namely ortho-xylene, meta-xylene and para-xylene, are important chemicals and find wide and varied application in industry. Ortho-xylene is a reactant for the production of phthalic anhydride. Meta-xylene is used in the manufacture of plasticizers, azo dyes, wood preservers, etc. Para-xylene upon oxidation yields terephthalic acid which is used in the manufacture of synthetic textile fibers.
As a result of the important applications to which the individual xylene isomers are subjected, it is often very important to be able to produce high concentrations of a particular xylene. This can be accomplished by converting a non-equilibrium mixture of the xylene isomers, which mixture is low in the desired xylene isomer, to a mixture which approaches equilibrium concentrations. Various catalysts and processes have been devised to accomplish the isomerization process. For example, it is well known in the art that catalysts such as aluminum chloride, boron fluoride, liquid hydrofluoric acid, and mixtures of hydrofluoric acid and boron fluoride can be used to isomerize xylene mixtures.
Industrially, isomerization of xylenes and conversion of ethylbenzene is performed primarily to produce para-xylene. A typical processing scheme for this objective comprises: (a) separating para-xylene from a C.sub.8 alkylaromatic mixture using, for example, molecular sieve technology, to obtain a para-xylene-rich stream and a para-xylene-depleted stream; (b) isomerizing the para-xylene depleted stream to near equilibrium in an isomerization reaction zone; and, (c) recycling the isomerization product to separation along with the fresh C.sub.8 alkylaromatic mixture.
The present invention is particularly concerned with the isomerization reaction step which may be used in an overall process directed to para-xylene production. An important parameter to consider in this isomerization reaction step is the degree of approach to xylene equilibrium achieved. It is desirable to run the isomerization process as close to equilibrium as possible in order to maximize the para-xylene yield. However, associated with this is a greater cyclic C.sub.8 loss due to side-reactions (cyclic C.sub.8 hydrocarbons include xylenes, ethylbenzene, and C.sub.8 napthenes.) The approach to equilibrium that is used is an optimized compromise between high C.sub.8 cyclic loss at high conversion (i.e. very close approach to equilibium) and high utility costs due to the large recycle rate of unconverted ethylbenzene, ortho-xylene, meta-xylene, and C.sub.8 napthenes which result from the hydrogenation of the C.sub.8 aromatics. The correlation of cyclic C.sub.8 loss versus the distance from xylene equilibrium is a measure of catalyst selectivity. Thus there is a strong incentive to develop a catalyst formulation which minimizes cyclic C.sub.8 loss while maximizing para-xylene yield.
Numerous catalysts have been proposed for use in xylene isomerization processes such as mentioned above. More recently, a number of patents have disclosed the use of crystalline aluminosilicate zeolite-containing catalysts for isomerization and conversion of C.sub.8 alkylaromatics. Crystalline aluminosilicates generally referred to as zeolites, may be represented by the empirical formula: EQU M.sub.2/n O.Al.sub.2 O.sub.3.xSiO.sub.2.yH.sub.2 O
in which n is the valence of M which is generally an element Group I or II, in particular, lithium, sodium, potassium, magnesium, calcium, strontium, or barium, and x is generally equal to or greater than 2. Zeolites have skeletal structures which are made up of three-dimensional networks of SiO.sub.4 and AlO.sub.4 tetrahedra, corner-linked to each other by shared oxygen atoms. Zeolites with high SiO.sub.2 /Al.sub.2 O.sub.3 ratios have received much attention as components for isomerization catalysts. Representative of zeolites having such light proportion of SiO.sub.2 include mordenite and the ZSM varieties. It is also known in the art that zeolites of the ZSM series can be prepared with gallium atoms substituted for aluminum atoms, for example, see U.S. Pat. No. 4,585,641. In addition to the zeolite component, certain metal promoters and inorganic oxide matrices have been included in isomerization catalyst formulations. Examples of inorganic oxides include silica, alumina, and mixtures thereof. Metal promoters such as Group VIII or Group III metals of the Periodic Table, have been used to provide a dehydrogenation functionality. The acidic function can be supplied by the inorganic oxide matrix, the zeolite, or both.
When employing catalysts containing zeolites for the isomerization of alkylaromatics, characteristics such as acid site strength, zeolite pore diameter, and zeolite surface area become important parameters to consider during formulation development. Variation of these characteristics in a way that reduces side-reactions, such as transalkylation, is required in order to achieve acceptable levels of cyclic C.sub.8 loss.
It has been found that, if a catalyst is formulated with the components, and in the manner set forth hereinafter, an improved process for the conversion of a non-equilibrium mixture of xylenes containing ethylbenzene is obtained.